The first thing that you must start with is to have a detailed plan on your existing hardware inventory, how you are going to deploy Hyper-V, what storage resources do you have.
Then next on the list is the software which you are going to run, plan the VMs with the resource requirements for each software.
These are just the first 2 steps in "sizing" or planing your environment, so you need to look deeper into what you're trying to do, there are no such general "sizing" recomandations, as it largely depends on what you are going to do etc etc.
